For image forming devices, particularly color photocopiers, color printers and other color image forming devices, multistep drum types (tandem type) capable of higher speed processing than a single drum type device are becoming the mainstream. In a tandem color image forming device, image forming units, for example, magenta, yellow, cyan and black, are arranged by color in the paper transport direction, and a color toner image is sequentially formed by the image forming units of each color.
The image forming units of each color are arranged so as to face a transfer belt serving as an image support body, and the toner image sequentially formed by the image forming units of each color is transferred to the transfer belt by first transfer means. The transfer belt is endlessly wound on a plurality of rollers, and the toner image on the transfer belt is transferred to a transfer material by secondary transfer means. Meanwhile, the transfer material is transported from a sheet feed unit to a transfer position via a pair of resist rollers.
In such an image forming process, rippling or meandering may occur in the moving direction of a transfer belt wound endlessly on the plurality of rollers. Transfer belt meandering may cause image distortion, as in a square image becoming a parallelogram image and misalignment of color registration. Further, transfer belt rippling may cause image deformation, voids or the like.
In order to obtain a high-quality image where the colors are accurately superimposed and there is no image distortion, stable drive and transport without the occurrence of transfer belt meandering or rippling are necessary. Thus, a method has been proposed in which ribs are provided on the back of the belt on both sides thereof and the ribs are brought into direct contact with the retaining rolls, thereby preventing meandering. Although such a method in which ribs are provided on both sides of the belt is effective for preventing meandering, if for some reason, such as a difference in peripheral length of the two belt sides or the degree to which the rolls are parallel, a large meandering does occur, the ribs on both sides of the belt may ride up on the rolls.
In addition, technology has been proposed in which a steering roll inclines in response to belt meandering, thus correcting the meandering. Technology has also been proposed in which meandering correction rolls will be changed depending on the belt meandering direction. However, with such configurations, a meandering correction roll needs to be added, not only making the mechanical configurations complicated but also increasing the costs of the overall device.
Therefore, it is an object of the present invention to provide a belt device capable of preventing belt meandering with a simple configuration.